In recent years, a technique for forming transistors using a semiconductor thin film (with a thickness of several nanometers to several hundreds of nanometers) which is formed over a substrate having an insulating surface has attracted attention. Transistors have been widely applied to electronic devices such as an IC and an electro-optical device, and their development as particularly switching elements of image display devices has been hastened.
Metal oxides have been known as materials having semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics are exemplified by tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A transistor in which a channel formation region is formed using such metal oxides having semiconductor characteristics has been known (for example, see Patent Documents 1 and 2).
Transistors to which oxide semiconductors are applied have relatively high field effect mobility. Thus, a driver circuit for a display device or the like can be formed using the transistor including the oxide semiconductors.
[Reference]
    [Patent Documents]    [Patent Document 1] Japanese Published Patent Application No. 2007-123861    [Patent Document 2] Japanese Published Patent Application No. 2007-096055